1. Field of the Invention
This invention relates to the use of conjugated polymers in electroluminescence materials, wherein the conjugated polymers comprise at least one repeating unit based on a heterospiro framework. Such compounds surprisingly have excellent thermal stability, improved solubility in organic solvents, improved film-forming properties and particularly good electroluminescence with high color purity.
2. Description of the Related Art
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains, and are incorporated herein by reference.
There is a great industrial need for large-area solid-state light sources for a series of applications, predominantly in the field of display elements, VDU technology and lighting engineering. What is required of these light sources cannot at present be achieved fully satisfactorily using any of the existing technologies.
As an alternative to conventional display and lighting elements such as incandescent lamps, gas discharge lamps and non-selfilluminating liquid crystal display elements, use has been made for some time of electroluminescence (EL) materials and devices such as light-emitting diodes (LEDs).
Apart from inorganic electroluminescence materials, low molecular weight organic electroluminescence materials and devices have also been known for about 30 years (see, for example, U.S. Pat. No. 3,172,862). However, until recently, such devices have been greatly restricted in their practical usability.
WO 90/13148 and EP-A 0 443 861 describe electroluminescence devices comprising a film of a conjugated polymer as a light-emitting layer (semiconductor layer). Such devices offer numerous advantages such as the opportunity of producing large-area, flexible displays simply and inexpensively. In contrast to liquid crystal displays, electroluminescence displays are selfilluminating and therefore require no additional backward lighting source.
A typical device as described in WO 90/13148 comprises a light-emitting layer in the form of a thin, dense polymer film (semiconductor layer), which comprises at least one conjugated polymer. A first contact layer is in contact with a first surface, a second contact layer is in contact with a further surface of the semiconductor layer. The polymer film of the semiconductor layer has a sufficiently low concentration of extrinsic charge carriers for charge carriers to be introduced into the semiconductor layer on application of an electric field between the two contact layers, with one contact layer becoming positive relative to the other and the semiconductor layer emitting radiation. The polymers used in such devices are conjugated. A conjugated polymer is a polymer having a delocalized electron system along the main chain. The delocalized electron system gives the polymer semiconducting properties and enables it to transport positive and/or negative charge carriers with high mobility.
In WO 90/13148, poly(p-phenylenevinylene) is used as polymeric material for the light-emitting layer and it is proposed that the phenyl group in such a material be replaced by a heterocyclic or a condensed carbocyclic ring system. In addition, poly(p-phenylene), PPP, is also used as electroluminescent material.
Although these materials give good results, the color purity, is unsatisfactory. Furthermore, it is virtually impossible to produce a blue or white emission using the polymers known hitherto.
Since, in addition, the development of electroluminescence materials, in particular those based on polymers, can in no way be regarded as concluded, the manufacturers of lighting and display devices are interested in a great variety of electroluminescence materials for such devices. This is, inter alia, also because only through study of the interaction of the electroluminescence materials with the other components of the devices can conclusions be drawn in respect to the quality of the electroluminescence material.